Consistently use @returns for returned data types in JSDoc comments

Sometimes we also used `@return`, but `@returns` is what the JSDoc
documentation recommends. Even though `@return` works as an alias, it's
good to use the recommended syntax and to be consistent within the
project.

src/core/annotation.js

@@ -36,7 +36,8 @@ class AnnotationFactory {
    * @param {Object} ref
    * @param {PDFManager} pdfManager
    * @param {Object} idFactory
-   * @return {Promise} A promise that is resolved with an {Annotation} instance.
+   * @returns {Promise} A promise that is resolved with an {Annotation}
+   *   instance.
    */
   static create(xref, ref, pdfManager, idFactory) {
     return pdfManager.ensure(this, '_create',
@@ -318,7 +319,7 @@ class Annotation {
    * @memberof Annotation
    * @param {number} flag - Hexadecimal representation for an annotation
    *                        characteristic
-   * @return {boolean}
+   * @returns {boolean}
    * @see {@link shared/util.js}
    */
   hasFlag(flag) {
@@ -780,7 +781,7 @@ class WidgetAnnotation extends Annotation {
    * @private
    * @memberof WidgetAnnotation
    * @param {Dict} dict - Complete widget annotation dictionary
-   * @return {string}
+   * @returns {string}
    */
   _constructFieldName(dict) {
     // Both the `Parent` and `T` fields are optional. While at least one of
@@ -826,7 +827,7 @@ class WidgetAnnotation extends Annotation {
    * @memberof WidgetAnnotation
    * @param {number} flag - Hexadecimal representation for an annotation
    *                        field characteristic
-   * @return {boolean}
+   * @returns {boolean}
    * @see {@link shared/util.js}
    */
   hasFieldFlag(flag) {

src/core/core_utils.js

@@ -98,7 +98,7 @@ const ROMAN_NUMBER_MAP = [
  * @param {number} number - The number that should be converted.
  * @param {boolean} lowerCase - Indicates if the result should be converted
  *   to lower case letters. The default value is `false`.
- * @return {string} The resulting Roman number.
+ * @returns {string} The resulting Roman number.
  */
 function toRomanNumerals(number, lowerCase = false) {
   assert(Number.isInteger(number) && number > 0,

src/core/evaluator.js

@@ -2095,7 +2095,7 @@ var PartialEvaluator = (function PartialEvaluatorClosure() {
     /**
      * Builds a char code to unicode map based on section 9.10 of the spec.
      * @param {Object} properties Font properties object.
-     * @return {Promise} A Promise that is resolved with a
+     * @returns {Promise} A Promise that is resolved with a
      *   {ToUnicodeMap|IdentityToUnicodeMap} object.
      */
     buildToUnicode(properties) {

src/core/fonts.js

@@ -732,7 +732,7 @@ var Font = (function FontClosure() {
    * private use area. This is done to avoid issues with various problematic
    * unicode areas where either a glyph won't be drawn or is deformed by a
    * shaper.
-   * @return {Object} Two properties:
+   * @returns {Object} Two properties:
    * 'toFontChar' - maps original char codes(the value that will be read
    * from commands such as show text) to the char codes that will be used in the
    * font that we build